1. Field of the Invention
The present invention relates to a digital filter coefficient setting apparatus and a digital filter coefficient setting method, and more particularly, to a digital filter coefficient setting apparatus for and a digital filter coefficient setting method of setting a digital filter in response to a specified filter coefficient element.
2. Description of the Related Art
The digital filter is designed to filter a digitally encoded input signal. The digital filter has characteristics such as, for example, a passband, i.e., a frequency range over which the power of the input signal is passed to the filter output with approximately unity gain, a stopband, i.e., a frequency range where the power of the input signal is attenuated to a level which can be considered virtually absent in the output signal, and a cut-off frequency, i.e., a frequency, at which the power of the output signal is half the power of the input signal, wherein the cut-off frequency is provided in a transition band between the passband and the stopband. The digital filter is capable of, for example, altering the magnitude, frequency, or phase response of a digital signal in accordance with the digital filter characteristics. The digital filter characteristics of the digital filter can be approximately defined by known equations with filter coefficients. This means that the digital filter characteristics of the digital filter can be approximately determined by substituting a predetermined number of values for the filter coefficients in the equations.
Up until now, there have been proposed a wide variety of digital filter coefficient setting apparatuses each of which is operative to set a digital filter in response to a specified filter coefficient element. The filter coefficient element herein used is intended to mean a value to be substituted for one of the digital filter coefficients in the aforementioned equations. The filter coefficient element determines one of the digital filter characteristics of the digital filter. The conventional digital filter coefficient setting apparatus of this type is disclosed in, for example, the Japanese Patent Laid-Open Publication No. S60-182815.
One typical example of the conventional digital filter setting apparatus, hereinlater referred to as a first conventional digital filter setting apparatus 900, is shown in FIG. 10(a) as comprising a digital signal processor 906, hereinlater referred to simply as “DSP 906”, equipped with a digital filter, which is operable to filter a digital signal in accordance with a digital filter characteristic varied in response to a filter coefficient element, a coefficient storing unit 902 for storing a predetermined number of filter coefficient elements respectively in association with a predetermined number of coefficient parameters, a parameter inputting unit 901 for inputting a specified coefficient parameter, a coefficient selecting unit 903 for searching and selecting a specified filter coefficient element corresponding to the specified coefficient parameter inputted by the parameter inputting unit 901 from among the predetermined number of filter coefficient elements stored in the coefficient storing unit 902, and a coefficient transferring unit 905 for transferring the specified filter coefficient element thus searched and selected by the coefficient selecting unit 903 to the DSP 906. The DSP 906 is operative to set the digital filter in response to the specified filter coefficient element thus received.
The first conventional digital filter setting apparatus 900 shown in FIG. 10(a), however, encounters a drawback that the first conventional digital filter setting apparatus 900 may not set the digital filter in response to a specified filter coefficient element resulting from the fact that the coefficient selecting unit 903 of the first conventional digital filter setting apparatus 900 can search and select a specified filter coefficient element corresponding to a specified coefficient parameter only from among the predetermined number of filter coefficient elements stored in the coefficient storing unit 902, but cannot search and select a specified filter coefficient element, which is not stored in the coefficient storing unit 902. Furthermore, the coefficient storing unit 902 is required to store a large number of filter coefficient elements respectively in association with a large number of coefficient parameters in order to make it possible for the coefficient selecting unit 903 to search and select a specified filter coefficient element corresponding to any possible specified coefficient parameter to be inputted by the parameter inputting unit 901.
Another typical example of the conventional digital filter setting apparatus, hereinlater referred to as a second conventional digital filter setting apparatus 910, is shown in FIG. 10(b) as comprising a DSP 906, equipped with a digital filter, which is operable to filter a digital signal in accordance with a digital filter characteristic varied in response to a filter coefficient element, a parameter inputting unit 901 for inputting a specified coefficient parameter, a coefficient calculating unit 914 for calculating a specified filter coefficient element on the basis of the specified coefficient parameter inputted by the parameter inputting unit 901, and a coefficient transferring unit 905 for transferring the specified filter coefficient element thus calculated by the coefficient calculating unit 914 to the DSP 906. The DSP 906 is operative to set the digital filter in response to the filter coefficient element thus received. The second conventional digital filter setting apparatus 910 thus constructed may calculate and obtain a specified filter coefficient element corresponding to any possible specified coefficient parameter to be inputted by the parameter inputting unit 901, and set the digital filter in response to the specified filter coefficient element thus calculated and obtained. This leads to the fact that the second conventional digital filter setting apparatus 910 shown in FIG. 10(b) can solve the aforesaid problem.
The second conventional digital filter setting apparatus 910, however, encounters another drawback that the second conventional digital filter setting apparatus 910 may delay in setting the digital filter equipped in the DSP 906 in response to a specified filter coefficient element resulting from the fact that the coefficient calculating unit 914 cannot calculate and obtain the specified filter coefficient element instantaneously. The present invention is made with a view to overcoming the previously mentioned drawbacks inherent to the conventional digital filter setting apparatuses.